1. Field of the Invention
The present invention relates to a magnetic float for a magnetic sensor such as a reed switch in a level meter or the like which is turned on or off as the magnetic float moves up and down.
2. Description of the Prior Art
In a level meter for detecting the residual quantity (or presence) of the brake oil of an automobile, for example, a reed switch and a magnetic float are used such that the reed switch is attached to a guide of a pipe or the like and that an annular magnetic float is movably disposed around the guide. As the magnetic float floating at the liquid surface moves up and down along the guide in accordance with the change in the liquid surface, the reed switch is turned on and off.
The conventional magnetic float used in such level meter is constructed, as shown in FIG. 5, by combining a sintered magnet 1 having a larger specific gravity with a float 2 of a foamed synthetic resin having a smaller specific gravity. The float 2 of a foamed synthetic resin is integrally molded by inserting an annular sintered magnet 1 in a die and injecting a mixture of a synthetic resin such as nylon, polypropyrene or acrylonitrile-butadiene rubber and a foaming agent to the outside of the sintered magnet 1 to foam the mixture.
This magnetic float of the prior art is produced by the insert molding process, in which the sintered magnet 1 is inserted into the die, so that its production is complicated to raise the production cost. From the standpoint of performance, the sintered magnet 1 is exposed to the outside and is liable to be corroded by the liquid and to damage the surfaces of other parts.
In case, moreover, the conventional magnetic float is to be used together with the reed switch as the level meter, iron powder 3 or the like contained in the liquid will stick to the sintered magnet 1 to reduce the internal diameter of the magnetic float, as shown in FIG. 6, because the sintered magnet 1 having a high density of surface magnetic flux is exposed to the inner side of the magnetic float, i.e., the guide side of the pipe or the like equipped with the reed switch. This makes it necessary to make the internal diameter b of the magnetic float sufficiently larger than the external diameter a of the guide such as the guide equipped with the reed switch so that the magnetic float is large-sized. Accordingly, the level meter is large-sized to make the level meter expensive and to widen the space for mounting the level meter.
Since, moreover, the sintered magnet 1 has a large specific gravity, the size of the float 2 has to be enlarged the more as the size of the sintered magnet 1 is enlarged. In case, therefore, it is difficult to enlarge the sintered magnet 1, and in case the residual quantity (or presence) of oil having a small specific gravity is to be detected, as shown in FIG. 7, the sintered magnet 1 has its shown vertical thickness t reduced to 3 to 4 mm. As a result, when the sintered magnet 1 is moved up and down relative to a reed, switch 6, the ON range (i.e., a distance between points in which the reed switch 6 is turned on and off) of the reed switch 6 is narrowed so that the position of each reed switch 6 has to be adjusted for producing the level meter.
Specifically, when the reed switch 6 is to be turned on by the sintered magnet 1, the magnetic flux coming from the shown vertical magnetic poles (having an N pole at its upper portion and an S pole at its lower portion) of the sintered magnet 1 acts upon outer end portions of contact members 7 and 8 of the reed switch 6 (i.e., the upper end portion of the upper contact member 7 and the lower end portion of the lower contact member 8) so that the respective two outer end portions of the contact members 7 and 8 are polarized to the S and N poles, whereby inner end portions of the contact members 7 and 8 (i.e., the lower end portion of the upper contact member 7 and the upper end portion of the lower contact member 8) attract each other to establish a conduction. If the sintered magnet 1 has a small thickness t, the distances from the magnetic poles of the sintered magnet 1 and the outer end portions of the contact members 7 and 8 are elongated because the internal diameter b of the sintered magnet 1 is sufficient. Even if the surface densities of the magnetic fluxes of the magnetic poles of the sintered magnet 1 are high, the densities of the magnetic fluxes at the outer end portions of the contact members 7 and 8 are drastically reduced to narrow the range in which the reed switch 6 is turned on by the sintered magnet 1. As a result, unless the position of each reed switch 6 is adjusted when the level meter is to be produced, a malfunction may arise. Thus, it is necessary to adjust the position of each reed switch 6.
Since the magnetic float for the magnetic sensor of the prior art has problems in the production process and in the performances, as has been described hereinbefore, it is difficult to drop the production cost, and other problems arise in the durability and the magnetic characteristics in case the magnetic float is to be assembled in the product such as the level meter together with the magnetic sensor such as the reed switch.